Every New Year's, about half of all Americans make resolutions, many of which center around eating less and exercising more. These are certainly worthy goals, considering about one-third of American adults are obese, and another one-third are overweight.

But sadly, the vast majority of us fail, and we blame it on a lack of willpower. Why aren’t losing weight and increasing exercise just as easy as remembering to brush our teeth, not overspending, or calling a parent more regularly? A closer look at human history and biology suggests that the fault—to a great extent—lies in our genes.

As I explain in my book, Too Much of a Good Thing: How Four Key Survival Traits Are Now Killing Us, we are the descendants of more than 10,000 generations of humans who survived through countless food shortages and famines by gorging themselves whenever food was available. The DNA they passed down to us includes genes that changed—or mutated—over those generations to make us crave sugars, proteins, and salt; assure our intestines absorb essentially everything we eat; and help us store any excess calories as fat, to be burned during future times of need.

Not surprisingly, some fat-storing genetic changes are more frequent among those of us whose ancestors came from places where calories were often scarce, such as the Pimas of the American Southwest. The ability to hoard fat was overwhelmingly beneficial for most of their history. But about 75 years ago, when food became plentiful and physical activity plummeted, that same trait turned into a major liability. Today’s Pimas have one of the highest rates of obesity and diabetes in the world. Unfortunately, the rest of us aren’t very far behind.

Making matters worse, our bodies are built to defend whatever weight we currently carry. When we lose weight—regardless of whether we are overweight or underweight—our hormones make us hungrier and slow down our metabolism so we burn fewer calories. Consequently, it’s much easier to avoid putting on weight than it is to lose that excess weight.

The story behind physical exertion is less well understood but probably similar. About 20 percent of us have a specific gene mutation that makes us more likely to seek thrills. The genetic drive for roaming and thrill-seeking served our forebears well when survival depended on hunting, gathering, and winning fights. In the modern era of cars, supermarkets, desk jobs, and relative peace, we can survive quite easily without moving very much, and most of us don’t compensate for this reduced activity by sufficiently increasing our leisure-time exercise. In other words, our actions aren't matching up with our genes.

Of course, some of us will buck the odds and be successful—we’ll eat less, exercise more, and shed those unwanted pounds. But for every remarkable success, many more dieters will “yo-yo” down and back up with little net success, or never even “yo” down in the first place.

I am not arguing against New Year’s resolutions or against trying to lose weight and improve your health. We can and should strive to be better, and losing weight and increasing exercise are two of the best ways to reduce the likelihood of diabetes, high blood pressure, and heart disease.

However, we must recognize that when we or others fail in these endeavors, it’s not primarily a sign of moral weakness—but rather the inability to overcome the modern downsides of the same genes that allowed our ancestors to perpetuate our species for 200,000 years.